The present invention relates to a novel method for the chlorination of a silicon compound. More particularly, the invention relates to a method, starting from a silicon compound having at least one hydrogen atom directly bonded to the silicon atom, for the preparation of a chlorinated silicon compound by selectively replacing a part or all of the silicon-bonded hydrogen atoms in the starting silicon compound with chlorine atoms.
As is well known in the technology of silicone products, silicon compounds or silane compounds having, per molecule, at least one silicon-bonded hydrogen atom and at least one silicon-bonded chlorine atom, such as trichlorosilane, methyl dichlorosilane, dimethyl chlorosilane, methyl phenyl chlorosilane and the like, are very useful. These silane compounds or, in particular, methyl dichlorosilane, dimethyl chlorosilane and the like are industrially prepared by the so-called "direct method" reported by E. G. Rochow, et al. in U.S. Pat. No. 2,380,995 and Journal of the American Chemical Society, volume 67, page 1772 (1945). Alternatively, trichlorosilane, which is obtained by the reaction of elementary silicon and hydrogen chloride, can be used as an intermediate from which these silane compounds can be derived.
When it is desired to introduce an ordinary organic group into the molecule of a silane compound in the preparation of these silane compounds, several methods are known in the prior art including:
(1) a method described in Organometallic Compounds of the Group IV Elements, by A. G. MacDiarmid, editor, volume 1, pages 106 to 536 (1968), in which the silicon-bonded chlorine atom in the silane compound is replaced with the organic group R by using an organometallic compounds, e.g., Grignard reagent, according to the reaction equation EQU .tbd.SiCl+RMgX.fwdarw..tbd.Si--R+MgXCl,
in which X is a halogen atom, and
(2) a method reported by J. L. Speier, et al. in Journal of the American Chemical Society, volume 79, page 974 (1957) utilizing the so-called hydrosilation reaction between the silicon-bonded hydrogen atom in the silane compound and an .alpha.-olefin compound of the formula CH.sub.2 .dbd.CH--R promoted by a platinum compound as a catalyst according to the reaction equation EQU .tbd.SiH+CH.sub.2 .dbd.CH--R.fwdarw..tbd.Si--CH.sub.2 CH.sub.2 --R.
The above described first method utilizing an organometallic compound, which is applicable to the synthesis of a silane compound having a silicon-bonded hydrogen atom and a silicon-bonded chlorine atom in a molecule, is not suitable for use when such an organometallic compound cannot be used as in the case where the silane compound has a halogen-substituted organic group so that the only method applicable thereto is the above described second method. This method of hydrosilation, however, is not without problems in respects of the low reaction velocity when the silane compound has two or more silicon-bonded hydrogen atoms in a molecule and uncontrollability of the number of the organic groups introduced into a molecule of the silane compound.
Accordingly, the substitution reaction between a silicon-bonded hydrogen atom and a silicon-bonded chlorine atom provides a possibility for the synthesis in general, of which the reducing reaction of a silicon-bonded chlorine atom Si--Cl into a silicon-bonded hydrogen atom Si--H comes first. The reducing reaction can be performed in several different ways including a method of reduction in a polar solvent such as amides, imides and the like with sodium borohydride NaBH.sub.4 or sodium hydride NaH as the reducing agent as disclosed in Japanese Patent Publication 55-34798, a method reported in Journal of Organometallic Chemistry, volume 18, page 371 (1969), in which dimethyl dichlorosilane is reacted with diethylamine to produce dimethyl (N,N-diethylamino) chlorosilane which is reduced with lithium aluminum hydride LiAlH.sub.4 to give dimethyl (N,N-diethylamino) silane followed by the substitution of chlorine atoms for the diethylamino groups and a method disclosed in a Russian journal Zhurnal Obschchii Khimii, volume 40, page 812 (1970), in which dimethyl dichlorosilane is reduced with sodium hydride in the presence of aluminum chloride. These methods, however, are each disadvantageous because the reaction procedure is very complicated requiring special skillfulness to cause a difficulty in the control of the reaction along with the poor selectivity of the reaction.
A still alternative method known is the method of partial chlorination of a polyhydrosilane compound. A most conventional chlorinating agent for this purpose is chlorine for the conversion of a silicon-bonded hydrogen atom Si--H into a silicon-bonded chlorine atom Si--Cl as is disclosed in Japanese Patent Publication 3-10636 and Japanese Patent Kokai 2-67289, 2-145591 and 2-157286. This method, however, is not applicable for the purpose of partial chlorination because the reaction proceeds too violently.
Other known methods for the substitution of a chlorine atom for a silicon-bonded hydrogen atom include a method reported by J. Curtice, et al. in Journal of the American Chemical Society, volume 79, page 4754 (1957) and by Y. Nagai, et al. in Journal of Organometallic Chemistry, volume 9, page 21 (1967) according to which a chlorine-substitution reaction takes place between a hydrosilane compound and carbon tetrachloride in the presence of benzoyl peroxide as a catalyst and a method reported by Y. Nagai, et al. in Kogyo Kagaku Zasshi, volume 71, page 112 (1968) by using palladium chloride as a catalyst. These methods are also not suitable when partial hlorination is desired of a silicon compound having a plurality of silicon-bonded hydrogen atoms in a molecule.